The present invention relates to a system for processing remote information, and more particularly, to a data modulator-demodulator, process and apparatus of a spread spectrum communication system transmitting and receiving information from a subscriber terminal (a credit card inquirer, a security monitor) connected in parallel with a subscriber line through a voice speech line regardless of main communication use, and especially, to a modulator-demodulator apparatus utilizing pseudo noise codes for spread spectrum modulation and demodulation for performing a high speed data transmission and reception regardless of concurrent vocal or speech use on the transmission.
In a remote communication using a transmitting medium such as a telephone line or a power line, it is difficult to completely embody a service for processing remote information irrelevant to a conventional communication service (such as a voice service in case of the telephone line) owing to technical difficulties.
In a case of telephone line use, especially, to prevent influence upon the existing voice service, an in-phase transmission system using a ground and a transmitting system using a non-voice band off set from a voice band have been researched.
A device typically found in data communications systems is the modem. A modem is a combination modulator and demodulator, and is often also referred to as a data set, line adaptor, modulator, or subset. Regardless of the name, the purpose of each of these components is to convert digital pulses into a form compatible and suitable for transmission over a communication system. The digital pulses that represent computer-related data are distorted and attenuated or weakened by telephone lines characteristics due to direct current and VHF components, contained in the pulses, but telephone channels are only designed to transmit frequencies from about 500 Hz to 3500 Hz.
Typically, a modem converts the digital data to analog signals to fit between the telephone channel frequencies by modulation and the data is recovered by demodulating the received analog signals. This allows the data to be transmitted by telephone at high speeds without undue distortion. Modems are characterized by, among other things, the modulation technique used (acoustic modems, integral modems, etc).
To perform the data communication without having influence upon the vocal speech service in the remote information processing system, a conventional data modulator-demodulator apparatus in the remote information processing system executes a data over voice (hereinafter, referred to as "DOV") communication for performing a FSK (Frequency Shift Keying) modulation or has used a longitudinal transmission system using a ground.
The modulator-demodulator apparatus of the DOV transmission system uses the FSK modulation system to use the band over the voice band.
There is a possibility however, of creating a problem of crosstalk since the modulator-demodulator apparatus of the DOV transmission system using the FSK modulation system transmits a high level signal to implement long-distance communication.
Moreover, the modulator-demodulator apparatus of the longitudinal transmission system should be connected perfectly to the ground from a remote place and in a transmitting speed respect, there has been a problem of a slow transmitting speed due to a difficulty of high frequency use.
Furthermore, the above mentioned conventional modulator-demodulator apparatus has the problem such as a weakness in data security owing to its own non-privacy function.
Spread spectrum modulation has been employed in order to avoid a wide range of communication problems, such as, for example, interference which occurs when there are multiple propagation paths between the transmitter and receiver. A typical modem tends to interfere with itself, due to such multiple propagation paths, via delayed reception of is own signal. Two fundamental types of spread spectrum systems are direct sequence and frequency hop systems. Direct sequence spread spectrum systems use phase modulation, e.g. binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK), to accomplish spreading, whereas frequency hop spread spectrum systems use a rapid changing of the carrier frequencies.
The implementation of spread spectrum systems requires the generation of spreading codes at both the transmitter and receiver, such as pseudorandom sequences. Typical pseudorandom sequences are M-series codes and Gold codes and are also known as pseudo noise codes.
One earlier technique, a Spread Spectrum Modem is described by Sherman M. Chow and Pok F. Lee in U.S. Pat. No. 4,481,640 uses a Hamming weighted filtered Barker code, along with the data to be transmitted, applied to a mixer to generate a spread spectrum signal in order to reduce or eliminate the effect of multipath fading. However, The Chow et al however, modem splits the digital data into even and odd bit data streams, having synchronous timing, and each pair of bits must be phase shifted by .pi./4 to encode clock timing into the signal for later clock recovery.
Kaoru Endo U.S. Pat. No 4,864,589 describes Spread Spectrum Power Line Communications between a plurality of slave units and a single master unit wherein the slave unit must detect whether the power line is already in use as a transmission path before the slave unit can transmit to the master unit. Endo's device uses Gold codes to identify the slave units and the Gold code is applied to an exclusive-OR gate along with the data to be transmitted in order to generate a spread spectrum signal.
A more recent efford in found in CSK Communication System in Kaoru Endo, et al. U.S. Pat. 5,132,986 which discusses intended improvements over the spread spectrum system using a pseudo noise code and a data stream processed by an exclusive-OR gate (such as in Endo's '589 patent. The Endo, et al. system is a code shift keying spread spectrum system using a pseudo noise code to transmit a "0" bit and the same pseudo noise code being phase shifted to transmit a "1" bit, the pseudo noise codes being M-series codes. We have observed however, that the CSK system requires the use of two PN (pseudo noise) correlators in the demodulator in order to re-obtain the transmitted data.